[jeffmould]

Was about to say almost the same thing. Without knowing the market, the app, the competition, the question can't be answered correctly. However, I would not take that one or two apps failed as a definitive sign there is no market. There could have been other forces at play such as timing, budget, quality of app, marketing, etc.. that all resulted in their failure.

As for the second question as to what percent of an app startup's success is tied to marketing? The answer would be 100%. That marketing can be as simple as telling your circle of friends and convincing them to tell their friends, but it has to start somewhere. A basic website, reaching out to bloggers/reporters, telling friends and family, that is all marketing. Without some form of marketing it will never succeed because nobody will ever hear about your app.